This application claims the priority of Japanese Patent Application No. 7-314227/1995 filed on Dec. 1, 1995, which is incorporated herein by reference.
1. Field of the Invention
This invention relates to a method of and an apparatus for foaming a viscous material which is particularly adapted for use as, e.g., a gasket to be formed at a job site or a fissure-filling sealant.
2. Description of the Background Art
FIG. 10 shows a schematic view of a circuit that constitutes a prior art apparatus 90 of the kind indicated above.
In the apparatus illustrated in FIG. 10, a sump 91 containing a viscous material is provided, and a pipe leads therefrom to a power mixer 94 by way of a primary pump 92. A gas bomb 93 is filled with compressed gas, which is fed to the power mixer 94 after pressure adjustment.
The power mixer 94 is connected to a suitable drive motor M. When the drive motor M is energized, the viscous material and the compressed gas are stirred under pressure. The mixture, which is fed through a tubular conduit 95, emerges from a nozzle 96. Nitrogen, carbon dioxide and air may be suitable for use as the compressed gas. The apparatus 90 is used for, e.g., applying a coat of viscous polymer such as hot melt adhesive (as disclosed in Japanese Laid Open Patent Application No. 63-264327).
The hot melt adhesive comprises a thermoplastic polymer, which is solid at ordinary temperatures. Although the hot melt adhesive is molten into a fluid state when it is heated, it resumes the solid state when it is cooled to room temperature. Consequently, the hot melt adhesive provides good adhesion of one component to another. The prior art apparatus takes advantage of the property of the hot melt adhesive which provides good adhesion immediately after cooling down before the gas mixed therein is dispersed.
In the apparatus 90, the viscous material and the compressed gas have to be heated and subjected to pressure feed to the power mixer 94. When the viscous material has a viscosity of 100,000 cps, the power mixer 94 is assumed to have an internal pressure of 100 Kg/cm.sup.2 or more. The compressed gas has to be put under higher pressure than the viscous material if the compressed gas is to be fed to the power mixer 94 simultaneously with the viscous material.
When the gas pressure is high, it is difficult to control the flow of the gas. A small error in the flow of the gas at the time of high pressure shows itself as a large error at the time of atmospheric pressure. For example, an error at the time of atmospheric pressure is 50 times as large as the error at the time of 50 Kg/cm.sup.2. A large dispersion of the mixing ratio of the viscous material to the compressed gas is caused thereby. The gas phase which is distributed as bubbles in the interior of the viscous material is rendered uneven.
In view of the above-described problems, one of the primary objects of the invention is to provide a method of and an apparatus for foaming a viscous material in which gas at low pressures can be mixed with the viscous material and the flow of the gas can be easily controlled so that the dispersion of the mixing ratio of the viscous material to the gas may be confined within a small tolerance and the bubbles may be uniformly distributed in the interior of the viscous material.